The advantages of such coatings are well-known. It is known since a long time that it is particularly useful to deposit TiN layers on parts that have to be surface hardened, for example for coating tools, to increase their resistance to wear. However, it has been noted that TiN layers are liable to deterioration with time due to oxidation. However, the introduction of aluminum in the TiN array provides a good resistance to oxidation because, during use, a thin Al.sub.2 O.sub.3 layer is formed and protects the hard coating, which largely increases the lifetime of the tool. This result is not obtained by a mixed deposit of TiN and AlN.
Thus, many searches have been made for implementing Ti.sub.1-x Al.sub.x N coatings. The known methods are mainly physical deposits such as reactive sputtering, possibly magnetron enhanced, and arc reactive coatings. These methods are not easy to implement, in particular due to the problem of making the sources. Additionally, sputtering type methods or projection type methods are not well adapted to the coating of parts having complex shapes.
Therefore, one has tried to find a deposition method of the CVD type. However, while it is known to make CVD deposition of TiN layers or AlN layers from titanium chloride and aluminum chloride in the presence of ammonia and/or nitrogen, it has always been considered that the simultaneous use of titanium chloride and aluminum chloride will provide mixed depositions of AlN and TiN and not to the deposition of an homogeneous layer of Ti.sub.1-x Al.sub.x N type phase.
Additionally, attempts to make CVD coatings from organo-metallic compounds of titanium and aluminum have been unsuccessful up to now.
One will more particularly consider the article of Sang-Hyeob Lee, Ho-Joon Ryoo and Jung-Joog Lee of the Seoul University, J. Vac. Sci. Technol., A 12(4), July-August 1994. In the introductory part of this article, the authors summarize the various known processes for forming Ti.sub.1-x Al.sub.x N coatings such as above-mentioned and indicate the difficulties that exist due to the fact that TiN has a NaCl type structure while AlN crystallizes in an hexagonal structure of the wurtzite type. However, the authors tried to find a new CVD method and claimed to obtain Ti.sub.1-x Al.sub.x N coatings by a plasma enhanced CVD method, using TiCl.sub.4, AlCl.sub.3, NH.sub.3, H.sub.2 and argon (forming the plasma) in an RF plasma or 15-200 watts. It would be emphasized that a plasma enhanced CVD deposition is not a simple operation and is not adapted to coat parts having complex shapes.
An object of the invention is to provide a method of CVD depositing Ti.sub.1-x Al.sub.x N layers from simple precursors, and without a plasma enhancement.